1. Field of the Invention
The invention relates to a valve drive for activation of gas exchange valves of internal combustion engines.
2. The Prior Art
It is known to operate gas exchange valves of an internal combustion engine variably with different opening and closing time points, as well as with different valve opening strokes. Such a valve control is described in German Patent Application No. DE 42 30 877 A1. In this connection, a camshaft block having two different cam contours is disposed on a camshaft so as to rotate with it, but in an axially displaceable manner. Depending on the axial position of the cam block, a cam contour stands in a functional connection with the stroke valve, by way of an intermediate element (transfer lever). The axial displacement of the cam block for changing the valve parameters takes place during the base circle phase, counter to the effect of a reset spring, by means of a pressure ring.
German Patent Application No. DE 35 20 859 A1 describes an internal combustion engine having at least one camshaft driven by a crankshaft, for activation of inlet and outlet valves. Two cams having different structures, in terms of their cam contour, and placed directly next to one another are disposed on the camshaft; these cams determine the opening and closing time point and the opening stroke in accordance with their configuration, taking the valve play into account. During passage through the cam base circle, which has the same shape in both cams, an adjustable intermediate piece is displaced, by way of a shift rod and a lever, in such a manner that optionally, one of the two cams can be brought into a functional connection with the valve.
German Patent Application No. DE 195 19 048 A1 describes a variable valve drive for an internal combustion engine, in which again, two cams having different structures, in terms of their cam contour, and placed directly next to one another are disposed on the camshaft. The change in cam engagement takes place by axial displacement of the camshaft with the cam situated on it.
Furthermore, a valve drive of an internal combustion engine is described in German Patent Application No. DE 195 20 117 C2, in which an axially displaceable cam block having at least two different cam paths is disposed on the camshaft, so as to rotate with it. The adjustment of the cam block takes place by way of an adjustment organ that is guided in the interior of the camshaft. The adjustment organ is displaced in the interior of the camshaft by a dual-action hydraulic or pneumatic piston/cylinder unit disposed on the face side of the camshaft. The adjustment organ is connected with an entrainment piece that penetrates an oblong hole disposed axially in the camshaft, and ends in a bore of the cam block.
German Patent Application No. DE 100 54 623 A1 describes a device for switching over a cam package on a camshaft, to activate gas exchange valves, in which the cam package is guided on the camshaft in an axially displaceable manner. The gas exchange valve is in a functional connection with different cam contours, in accordance with the position of the cam package. The adjustment of the cam package takes place by way of a setting element in interaction with a gate track. The setting element is a pin that can be displaced radially to the outside, which interacts, in the moved-out state, with at least two gate tracks formed in a guide part disposed around the cam package by approximately 180°.
A disadvantage of the cited prior art is the great need for construction space that is required to adjust the cam block. These solutions can therefore be used only in the case of relatively large cylinder distances, so that the corresponding components can be accommodated. Another disadvantage is the great mass forces that occur during the setting process, which are required for displacing the cam blocks or the adjustment organs. Switching to a corresponding cam contour can generally take place only cylinder-selectively with the solutions named in the prior art. Valve-selective switching is not possible.
A significant disadvantage of DE 100 54 623 A1 is that in order to switch to a different cam contour, the pin has to be moved out of the camshaft and tracked into an axially displaceable shift gate. After the switching process, the pin has to be moved back in again. This design is very parts-intensive and production-intensive, and there is the risk of damage to the camshaft, resulting from incorrect shifting of the pin. A further disadvantage is that the engine speed of rotation is limited because of the required adjustment time of the pin. Furthermore, the adjustment is dependent on the oil pressure that is present.
German Patent Application No. DE 10 2004 033 798 A1 describes a valve stroke switching mechanism for gas exchange valves in an internal combustion engine, between two different cam contours, by means of a shift gate disposed on the housing of the internal combustion engine so as not to rotate, but in an axially displaceable manner. The shift gate partly surrounds the camshaft and is provided with a gate groove that widens opposite to the direction of rotation of the camshaft, the side walls of which groove each form a gate flank that can be brought reciprocally into a functional connection with a contact surface disposed on an axially displaceable second cam contour, on both sides, laterally, in order to switch the valve stroke. During valve stroke switching, the axially displaceable second cam contour is either pushed over the cam contour of the cam that is firmly connected with the camshaft, by means of the shift gate, or pushed away from the cam contour, so that optionally, two different cam contours can be brought into a functional connection with the gas exchange valve.